1. Field of the Invention
The present invention relates to economical inertial navigation units (IMU's) for short range, relatively low accuracy guidance applications such as munitions. More particularly, this invention pertains to a multisensor of the type that employs paired triads of accelerometers mounted upon counteroscillating platforms for directly measuring linear accelerations and for determining rotation rates from Coriolis forces with respect to a three-axis system.
2. Description of the Prior Art
IMU's measure space-dependent rotations and accelerations with respect to orthogonal space axes. Their designs are beset by numerous difficulties as this requires the simultaneous measurement of six independent variables. For example, gyroscopes of the ring laser and fiber optic type require a lasing cavity dedicated to each input axis, mandating a total of three lasing cavities, an expensive undertaking, for obtaining three of the six measurements required of an IMU. (An example of a laser device for measuring rotation about three axes is shown in United States patent, property of the assignee herein, U.S. Pat. No. 4,795,258 of Martin entitled "Nonplanar Three-Axis Ring Laser Gyro With Shared Mirror Faces".) IMU's employing spinning wheel gyros must deal with such gyros' limitation to measurement of rotation with respect to two axes are limited to two axes of measurement, necessitating the use of an additional drive mechanism for the third input axis. Again, this does not in any way account for the additional complication introduced by the remaining measurements of accelerations with respect to the axes.
Simplicity and economy are particularly significant in the design of IMU's for munitions guidance and like applications. Such uses are characterized by non-reusable payloads, limited flight durations and only moderate accuracy requirements. One economical type of system for measuring both rotation rates and linear accelerations with reference to a set of three orthogonal axes is a multisensor mechanism taught in a series of United States patents, also the property of the assignee herein (U.S. Pat. No. 4,996,877 entitled "Three Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillator"; U.S. Pat. No. 5,007,289 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator"; and U.S. Pat. No. 5,065,627 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator.") The devices disclosed in the referenced patents employ piezoelectric drive mechanisms for driving a pair of counterbalanced platforms to oscillate out-of-phase about a common axis within a housing or case. Accelerometers housed in a vacuum to avoid the effects of gas damping are mounted in a tilted manner (for measuring variables in orthogonal planes) to radially-directed elements of the platforms provide measures of both linear acceleration and rotation. The latter (rotation) values are derived from the (Coriolis) forces sensed by the accelerometers at the resonant frequency of the counteroscillating structure.
The oscillatory motions of the rotors of the multisensors taught by the above-identified patents are coupled to one another through the case that houses the mechanism. Each rotor comprises three radially-directed rotor arms. An accelerometer is fixed to each rotor arm. The rotor arms alternate with rotor platforms, each including three radially-directed webs. Piezoelectric elements are mounted to either side of the outer webs. The elements are appropriately-poled so that an input drive voltage signal simultaneously induces compression and tension at the opposite surfaces to cause predetermined bending of the webs that results in oscillation of the rotors. The central web is relatively stiff. Such stiffness is the major factor that determines the natural or resonant frequency of the rotor.
Each rotor is bolted only to the case for support. As a result, the case provides the only path for transferring energy between the oscillating rotors. As mentioned earlier, measurement of rotation rate through sensing of Coriolis acceleration relies upon the demodulation of an output signal whose frequency is equal to the resonant frequency of the paired rotors. A single resonant frequency is assumed. The above-described design is subject to factors that can complicate the measurement of rotation rate to a significant extent. Many of such complications follow from the only-indirect coupling of energy (i.e. through the case) between the paired rotors.
Numerous arrangements can act to weaken the already-indirect coupling of energy. For example, many multisensor applications require that the case be hard-mounted to a body. In such applications, the mechanical impedance of the outside world is integrated into the coupling of the rotors so that the transfer of energy between the oscillating rotors is subject to attenuation in complex, and sometimes-unforeseen, ways. Thus, the accuracy of rotation rate measurement can vary as a function of application and changes in mechanical impedance.
Solutions to problems relating to weakly-coupled rotors to overcome the leakage of energy and problems related to differential rotor frequencies are quite complex and often expensive to implement. One solution, adjusting the relative amplitudes of the rotor drive voltages, can introduce bias effects, complicate system electronics, etc. Another solution is to mount the multisensor case on isolators so that the device is no longer hard-mounted to the outside world. While essentially solving the problems of external impedances, isolation-mounting multiplies mechanical complexity, size and cost, often to a significant extent.